1. Field of the Invention
The present invention relates to a resin encapsulated semiconductor device and, more particularly, to a resin encapsulated pin grid array having a heat radiating structure and a method of manufacturing the same.
2. Description of the Prior Art
A pin grid array (to be referred to as a PGA hereinafter) will be described below as an example of a resin encapsulated semiconductor device.
Recently, in order to widen a range of applications of the PGA having an IC chip thereon, the IC chip is replaced with another to obtain another function, and a ceramic has been used for a substrate of the PGA for this purpose.
A ceramic substrate has a good insulating property and therefore has high reliability as a product. However, the ceramic substrate shrinks because wiring patterns are formed by printing and baking. Therefore, it is difficult to increase the number of wiring patterns or to achieve micropatterning. In addition, if the number of patterns is increased, the substrate size is increased accordingly. Furthermore, the ceramic substrate is expensive as parts.
As a substrate which can take place of the ceramic substrate, a PGA using a resin substrate (to be referred to as a "resin substrate PGA" hereinafter) which can achieve micropatterning and is inexpensive has been developed. The present applicant proposed, in Japanese Patent Application No. 61-87081, a packaging structure in which the upper and side surfaces of a resin substrate having a plurality of contact pins on its lower surface and an IC chip on its upper surface are completely covered with a molded resin.
That is, a PGA having the packaging structure (to be referred to as a resin encapsulated PGA hereinafter) proposed in the above patent application has a packaging structure in which the upper and side surfaces of a resin substrate 2 having a plurality of contact pins 20 on its lower surface and an IC chip 1 on its upper surface are completely covered with an injection-molded resin 6.
However, although the PGA using the above resin substrate 2 can be made more compact at lower cost by micropatterning than the PGA using the ceramic substrate, it has not been widely used yet because of its poor heat radiation property.
That is, an IC to be mounted on the PGA is an LSI having a large chip size. Therefore, an amount of heat generated by an operation current is large, and a temperature of the LSI is increased if the generated heat is not rapidly radiated outside the package. In this case, a read speed of the LSI is reduced, or in the worst case, the LSI is thermally destroyed.
The heat radiation properties of the ceramic substrate PGA and the resin substrate PGA will be compared below. First, a heat conductivity of ceramic as a package material is 4.times.10.sup.-2 cal/cm.multidot..degree.C.multidot.sec, while those of a resin substrate and a molded resin which constitute the resin substrate PGA are 4.5.times.10.sup.-4 cal/cm.multidot..degree.C..multidot.sec and 2.times.10.sup.-3 cal/cm.multidot..degree.C..multidot.sec, respectively. That is, as for the heat conductivities of the packaging materials, that of the resin substrate PGA is 1/10 to 1/100 that of the ceramic substrate PGA. Therefore, heat generated in an IC chip of the ceramic substrate PGA is rapidly radiated through a ceramic substrate, a ceramic frame, and a ceramic cover. However, in the resin substrate PGA, a heat radiation amount is small because the resin material is used, and therefore a temperature of the IC chip is increased accordingly.
As described above, in order to widely spread the resin substrate PGA, its heat radiation property must be improved to a level closer to that of the ceramic substrate PGA. Therefore, conventional techniques will be described below in terms of improvements in the heat radiation property.
First, heat radiating paths of the PGA shown in FIG. 1 will be described. Heat generated from the IC chip 1 is radiated toward the lower surface through the resin substrate 2 and toward the upper surface through the injection-molded resin 6.
As for the path toward the upper surface, when a metal cap 5 is adhered on the upper surface of the encapsulating resin as indicated by an alternate long and dashed line in FIG. 1, the metal cap 5 serves as a heat radiating plate to significantly improve the heat radiation property. This phenomenon is not limited to the PGA. For example, a semiconductor which generates heat such as a power transistor having a conventional resin encapsulated structure has a metal heat radiation fin on the upper surface of the encapsulating resin to achieve the same effect.
As for the path toward the lower surface, no improvement has been made as a PGA as described above. However, heat radiating structures used in a resin substrate having a conventional semiconductor as shown in FIGS. 2 and 3 are known.
That is, in the structure shown in FIG. 2, a through hole 2a is formed in the resin substrate 2 at a position on which the IC chip 2 is mounted, and a metal vessel 50 formed by drawing is embedded in the through hole 2a. The IC chip 1 is placed in the vessel 50 to radiate heat generated from the IC chip 1 toward the lower surface through the vessel 50.
In the structure shown in FIG. 3, a metal plate 60 is adhered on the lower surface of the resin substrate 2 to cover the opening of the through hole 2a, and the IC chip 1 is placed on the metal plate 60. As a result, heat generated from the IC chip 1 is radiated toward the lower surface through the metal plate 60.
The above conventional techniques can be applied to the heat radiating structure of the resin substrate PGA. Of these conventional techniques, the heat radiating structure shown in FIG. 1 in which the metal cap 5 is adhered on or pushed in the encapsulating resin 6 has the following drawback.
That is, the heat radiation property toward the upper surface obtained by the metal cap 5 depends on an adhesion strength between the surface of the encapsulating resin 6 and the inner surface of the metal cap 5. However, if the metal cap 5 is adhered on or pushed in the encapsulating resin 6 as described above, the adhesion strength changes in accordance with surface precisions of the inner surface of the metal cap 5 and the surface of the encapsulating resin 6 or variations in amount of an adhesive and the like. As a result, a uniform heat radiation property cannot be obtained.
Furthermore, in the structure in which a heat radiating member such as the metal cap 5 or the heat radiation fin is mounted by adhering or pushing, a mounting height of the heat radiating member changes in accordance with a mounting state or a variation in amount of an adhesive. Therefore, a uniform outer shape as the PGA cannot be obtained. This poses a problem in terms of an outer appearance and prevents a high packing density in a limited space.